Air conditioner and method of controlling the same

ABSTRACT

An air conditioner includes a plurality of compressors, a common intake pipe, and a plurality of oil equalizing pipes that connect the compressors to the common intake pipe such that oil existing above a reference oil level in the compressors can be transferred to the common intake pipe. An oil equalizing valve provided on at least two of the oil equalizing pipes controls oil flowing through the oil equalizing pipes, where at least two of the oil equalizing valves can be opened together during performance of oil equalizing operation such that amounts of the oil in at least two compressors can be equalized.

This application claims priority from Korean Patent Application No. 10-2009-0012519 filed on Feb. 16, 2009, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates to an air conditioner, and more particularly, to an air conditioner and method of controlling the same, which is directed to solving oil imbalance between a plurality of compressors.

2. Description of the Related Art

Generally, an air conditioner is an appliance that cools or heats an indoor space by performing heat-exchange between refrigerant and indoor air using a refrigeration cycle in which the refrigerant is compressed, condensed, expanded, and evaporated. Air conditioners are classified into cooling air conditioners that supply cool air to an indoor space by operating the refrigeration cycle in only one direction and cooling-and-heating air conditioners that supply cool or hot air indoors by selectively operating the refrigeration cycle in one of two directions.

Air conditioners are further classified according to the connection structure of their indoor and outdoor units into standard air conditioners that have one indoor unit connected to one outdoor unit, and multi-type air conditioners that have a plurality of indoor units connected to at least one outdoor unit.

A typical multi-type air conditioner is used for selectively air-conditioning a plurality of separated spaces in a building by selectively operating as many compressors as necessary depending on the overall air conditioning load.

When multiple compressors operate, oil flows into the respective compressors along with the refrigerant. At this point, there is a difference in the amount of oil between the compressors according to the operating state of each compressor. That is, there may be compressors to which oil is excessively supplied and compressors to which oil is insufficiently supplied. Specifically, limitations such as reduced air conditioning efficiency may arise when oil is insufficiently supplied to compressors, which may burn out, generate excessive noise, and/or be deteriorated in performance. Therefore, it is critical to supply oil equally to multiple compressors.

SUMMARY

Accordingly, the present disclosure is directed to an air conditioner and method of controlling the same that substantially obviate one or more problems due to limitations and disadvantages of the related art.

One object is to provide an air conditioner and method of controlling the same, which can prevent oil from being excessively or insufficiently supplied to each of a plurality of compressors.

Another object is to provide an air conditioner and method of controlling the same, which can equally supply oil to each of a plurality of compressors.

Additional advantages, objects, and features will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided an air conditioner including a plurality of compressors, a common intake pipe, and a plurality of oil equalizing pipes that connect the compressors to the common intake pipe such that oil existing above a reference oil level in the compressors can be transferred to the common intake pipe. An oil equalizing valve provided on at least two of the oil equalizing pipes controls oil flowing through the oil equalizing pipes, where at least two of the oil equalizing valves can be opened together during performance of oil equalizing operation such that amounts of the oil in at least two compressors can be equalized.

In another aspect, there is provided an air conditioner including a plurality of compressors, a common intake pipe, and a plurality of oil equalizing pipes that connect the compressors to the common intake pipe such that oil existing above a reference oil level in the compressors can be transferred to the common intake pipe. A control unit is configured to control flow of oil in at least two oil equalizing pipes and to sense temperature at an output port side and an oil equalizing pipe side of at least two compressors that are operating.

In a further another aspect, there is provided an air conditioner including a plurality of compressors, a common intake pipe, and a plurality of oil equalizing pipes that connect the compressors to the common intake pipe such that oil existing above a reference oil level in the compressors can be transferred to the common intake pipe. A control unit is configured to open oil equalizing valves of at least two oil equalizing pipes connected respectively to at least two compressors that are operating for a predetermined time when degrees of discharge heat of at least two of the compressors that are operating are equal to or greater than a reference value.

It is to be understood that both the foregoing general description and the following detailed are exemplary and explanatory and are intended to provide an explanation of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:

FIG. 1 is a schematic diagram of an air conditioner according to an embodiment of the present invention;

FIG. 2 is a detailed diagram of an outdoor unit depicted in FIG. 1;

FIG. 3 is a schematic diagram illustrating devices that are directed to equally supplying oil to a plurality of compressors depicted in FIG. 2;

FIG. 4 is a block diagram of control system according to an embodiment of the present invention;

FIG. 5 is an exemplary graph illustrating a relationship between an amount of oil flowing into compressors and a value (Td−Tpipe); and

FIG. 6 is a flowchart illustrating a method of controlling an air conditioner according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Advantages and features, and implementation methods thereof will be clarified through following embodiments described with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the principle of the present invention to those skilled in the art. like reference numerals refer to like elements throughout.

FIG. 1 is a schematic diagram of an air conditioner according to an embodiment of the present invention, and FIG. 2 is a detailed diagram of an outdoor unit depicted in FIG. 1. Referring to FIGS. 1 and 2, an air conditioner of an embodiment of the present invention includes a plurality of outdoor units 100 and a plurality of indoor units 200. It is sufficient to have at least one indoor unit 100 and at least one outdoor unit 200, and thus the number of the indoor and outdoor units 100 and 200 are not limited to those shown in the figure. A high-low pressure common tube 117 may be installed between the outdoor units 100.

The outdoor unit 100 includes a plurality of compressors 120 a, 120 b, and 120 c that are interconnected by a refrigerant pipe 110, an outdoor heat exchanger 176, an outdoor expansion valve 171, and a super cooler 180. Pressure sensors 175 and 177 for measuring pressure of refrigerant flowing along the refrigerant pipe 110 may be installed on the refrigerant pipe 110. A solenoid valve 174 for adjusting the flow of the refrigerant may be installed on the refrigerant pipe 110.

The compressors 120 a, 120 b, and 120 c compress low temperature/low pressure refrigerant into high temperature/high pressure refrigerant. The compressors 120 a, 120 b, and 120 c may be variously structured. For example, inverter type compressors or constant speed compressors may be used as the compressors 120 a, 120 h, and 120 c. The compressors 120 a, 120 b, and 120 c are connected to the refrigerant common intake pipe, 111 in parallel. That is, the compressors 120 a, 120 b, and 120 c are connected to the refrigerant common pipe 110 in parallel by respective refrigerant intake pipes 111 a, 111 b, and 111 c. An accumulator 162 may be provided to prevent the liquid-phase refrigerant from flowing into the compressors 120 a, 120 b, and 120 c.

The compressors 120 a, 120 b, and 120 c compress the refrigerant and discharge the compressed refrigerant to respective refrigerant discharge pipes 112 a, 112 b, and 112 c. Hereinafter, compressor ports connected to the refrigerant discharge pipes 112 a, 112 b, and 112 c will be referred to as output ports. Pressure switches 132 a, 132 b, and 132 c may be provided on the respective refrigerant discharge pipes 112 a, 112 b, and 112 c to adjust pressure of the refrigerant that is being discharged.

Oil contained in the refrigerant discharged to each of the refrigerant discharge pipes 112 a, 112 b, and 112 c is separated from the refrigerant by each of oil separators 140 a, 140 b, and 140 c. The separated oil is recovered to the compressors 120 a, 120 b, and 120 c through respective oil recovery pipes 113 a, 113 b, and 113 c. That is, the refrigerant compressed by the compressors 120 a, 120 b, and 120 c contains the oil to smoothly operate the compressors 120 a, 120 b, and 120 c. The oil separators 140 a, 140 b, and 140 c separate the oil from the refrigerant and return the separated oil to the compressors 120 a, 120 b, and 120 c through the oil recovery pipes 113 a, 113 b, and 113 c so that the oil in the compressors 120 a, 120 b, and 120 c remains at a constant level. At this point, the oil recovery pipes 113 a, 113 b, and 113 c connect the oil separators 140 a, 140 b, and 140 c to respective compressor shells that are provided in the respective compressors 120 a, 120 b, and 120 c so as to collect the oil in the shells.

When an amount of the oil collected in the compressor shells increases to or above a reference oil level from which the compressors 120 a, 120 b, and 120 c can normally operate, the oil existing above the reference oil level is transferred toward the refrigerant common intake pipe 111. To realize this, oil equalizing pipes 114 a, 114 b, and 114 c that connect the compressor shells of the respective compressors 120 a, 120 b, and 120 c to the refrigerant common intake pipe 111 are provided. Capillary tubes 137 a, 137 b, and 137 c may be respectively provided on the oil equalizing pipes 114 a, 114 b, and 114 c.

A four-way valve 172 that is a directional control valve functions to guide the refrigerant compressed in the compressors 120 a, 120 b, and 120 c to the outdoor heat exchanger 176 in a cooling mode and to the indoor heat exchanger 220 in a heating mode. The four-way valve 172 connects into state A in the cooling mode and into state B in the heating mode.

The outdoor heat exchanger 176 is generally disposed at an outdoor space. The refrigerant heat-exchanges with the outdoor air while passing through the outdoor heat exchanger 176. The outdoor heat exchanger 176 functions as a condenser in the cooling mode and as a vaporizer in the heating mode. The outdoor expansion valve 171 expands the refrigerant directed from the indoor unit 200 in the heating mode. A blower fan 178 may be provided to discharge heat generated by the heat-exchange between the outdoor air and the refrigerant flowing along the outdoor heat exchanger 178 to an external side.

A first bypass pipe 179 is provided so that the refrigerant passing through the outdoor heat exchanger 176 can bypass the outdoor expansion valve 171 in the cooling mode. A check valve 173 for selectively checking the first bypass pipe 179 is provided. The check valve 173 is opened in the cooling mode.

The super cooler 180 includes a super cooling heat exchanger 182, a second bypass pipe 181, and a super cooling expansion valve 184. In the cooling mode, the second bypass pipe 181 directs the refrigerant flowing from the outdoor heat exchanger 176 toward the indoor unit 200 along a liquid pipe 115 to the super cooling expansion valve 184.

In the cooling mode, the super cooling expansion valve 184 expands the refrigerant passing through the second bypass pipe 181. Although there is a variety of types of super cooling expansion valves, a linear expansion valve may be used as the supper cooling expansion valve 184 considering convenience in use and control. The refrigerant that is condensed while passing through the outdoor heat exchanger 176 is super-cooled by heat-exchanging with the refrigerant that expands and is cooled down by the super cooling expansion valve 184 in the super cooling heat exchanger 182 while flowing toward the indoor units 200 along the liquid tube 115 and is then directed to the indoor units 200. Meanwhile, the refrigerant passing through the second bypass pipe 181 heat-exchanges in the super cooling heat exchanger 182 and is then transferred to the accumulator 162.

In this embodiment, each of the indoor units 200 includes an indoor expansion valve 210, an indoor heat exchanger 220, and an indoor flow fan 230 directing heat-exchanged air to the indoor space. The air conditioner may have one or more indoor units 200, and has four indoor units 200 in this embodiment.

The indoor heat exchanger 220 is generally disposed at the indoor space. The refrigerant passing through the indoor heat exchanger 220 heat-exchanges with the indoor air. The indoor heat exchanger 220 functions as a vaporizer in the cooling mode and as a condenser in the heating mode.

The indoor expansion valve 210 is a device for expanding the refrigerant that is supplied in the cooling mode. Although there is a variety of types of indoor expansion valves, a linear expansion valve may be used as the indoor expansion valve 210 considering convenience in use and control. The opening of the indoor expansion valve 210 may be adjusted depending on an air conditioning load of the indoor space where the indoor unit 210 is installed, and an air conditioning capacity of the indoor unit 200.

The following will describe the flow of the refrigerant in the cooling mode of the above-described air conditioner.

The high temperature/high pressure gas-phase refrigerant discharged from the compressors 120 a, 120 b, and 120 c is directed into the outdoor heat exchanger 176 via the four-way valve 172. The refrigerant is condensed in the outdoor heat exchanger 176 by heat-exchanging with the outdoor air. The refrigerant passing through the outdoor heat exchanger 176 is super-cooled while passing through the super cooler 180 and is then directed into the indoor unit 200 through the liquid pipe 115. At this point, the refrigerant in the second bypass pipe 181 of the super cooler 180 is directed to the accumulator 162.

Meanwhile, the refrigerant directed into each of the respective indoor units 200 expands by the indoor expansion valve 210 that is opened by a predetermined degree of opening and is vaporized in the indoor heat exchanger 220 by heat-exchanging with the indoor air. The vaporized refrigerant is transferred to the accumulator 162 through an air pipe 116 via the four-way valve 172. Hence, the refrigerant collected through the second bypass pipe 181 of the super cooler 180 and the refrigerant transferred from the indoor unit 200 are collected together in the accumulator 162. The collected refrigerant is supplied to the respective compressors 120 a, 120 b, and 120 c through the refrigerant common intake pipe 111, thereby continuously realizing a cooling cycle.

The following will describe the flow of the refrigerant in the heating mode of the above-described air conditioner.

The high temperature/high pressure gas-phase refrigerant discharged from the compressors 120 a, 120 b, and 120 c is directed into the indoor unit 200 through the air pipe 116 via the four-way valve 172. The refrigerant directed into the indoor unit 200 is condensed by heat-exchanging with the indoor air while passing through the indoor heat exchanger 220 and is then directed into the outdoor unit 100 through the liquid pipe 115, after which the refrigerant expands while passing through the indoor expansion valve 171 and is then vaporized by heat-exchanging with the outdoor air while passing through the outdoor heat exchanger 176. The vaporized refrigerant is directed to the respective compressors 120 a, 120 b, and 120 c via the four-way valve 172, accumulator 162, and refrigerant common intake pipe 111, thereby continuously realizing a heating cycle.

The following will describe the flow of the oil in the above-described air conditioner.

The refrigerant is compressed by the compressors 120 a, 120 b, and 120 c and discharged to the refrigerant discharge pipes 112 a, 112 b, and 112 c through the respective output ports. The oil separators 140 a, 140 b, and 140 c separate the oil from the refrigerant and return the oil to the compressors 120 a, 120 b, and 120 c through the respective oil recovery pipes 113 a, 113 b, and 113 c. At this point, since the oil recovery pipes 113 a, 113 b, and 113 c are structured to respectively connect the oil separators 140 a, 140 b, and 140 c to the compressors 120 a, 120 b, and 120 c, the redirection of the refrigerant into input ports of the compressors 120 a, 120 b, and 120 c through the respective oil recovery pipes 113 a, 113 b, and 113 c can be prevented by the compressors 120 a, 120 b, and 120 c and thus there is an effect that the compression efficiency and the coefficient of performance of the system can be improved.

In the above-described structure, since the oil separated by the oil separators 140 a, 140 b, and 140 c is, however, redirected into the corresponding compressors 120 a, 120 b, and 120 c, a compressor lacking oil continues suffering from a shortage of oil and a compressor having excessive oil continues to have excessive oil. In order to discharge the oil existing above the reference oil level in the compressor shells, oil equalizing ports are processed in the compressor shells and connected to the oil equalizing pipes 114 a, 114 b, and 114 c.

The oil existing above the reference oil level in the compressors 120 a, 120 b, and 120 c is dispensed to the refrigerant common intake pipe 111 through the oil equalizing pipes 114 a, 114 b, and 114 c. The oil dispensed to the refrigerant common intake pipe 111 together with the refrigerant is supplied again to the respective compressors 120 a, 120 b, and 120 c. The oil equalizing process between the compressors 120 a, 120 b, and 120 c will be described in more detail later.

FIG. 3 shows major parts of FIGS. 1 and 2, illustrating devices for equalizing oil between the compressors 120 a, 120 b, and 120 c. FIG. 4 is a block diagram of a control system according to an embodiment of the present invention. FIG. 5 is a graph illustrating a relationship between an amount of oil flowing into compressors and a value (Td−Tpipe).

The following will describe the oil equalizing process between the compressors 120 a and 120 b on the assumption that the compressors 120 a and 120 b are operating, the compressor 120 c stops operating, the compressor 120 a is in an oil excessive state, and the compressor 120 b is in an oil shortage state.

When the oil equalizing operation starts, a control unit 150 opens the oil equalizing pipes 114 a and 114 b. The oil existing above the reference oil level and collected in the compressor 120 a is directed to the common intake pipe 111 through the oil equalizing pipe 114 a. Meanwhile, since discharge pressure of the compressor 120 b is applied to the compressor shell of the compressor 120 b through the oil recovery pipe 113 b, the refrigerant is directed to the oil equalizing pipe 114 b.

When the oil equalizing operation is continued, the refrigerant containing the oil is dispensed to the compressors 120 a and 120 b that are operating through the common intake pipe 111. Accordingly, an amount of the oil of the compressor 120 b that is in the oil shortage state gradually increases and an amount in the compressor 120 a that is in the oil excessive state gradually decreases, thereby equalizing the oil between the compressors 120 a and 120 b.

In order to measure the temperature of the refrigerant discharged from the compressors 120 a, 120 b, and 120 c, first temperature sensors 131 a, 131 b, and 131 c are provided on the discharge pipes 112 a, 112 b, and 112 c and second temperature sensors 133 a, 133 b, and 133 c are provided on the oil equalizing pipes 114 a, 114 b, and 114 c.

The control unit 150 controls the first temperature sensors 131 a, 131 b, and 131 c, second temperature sensors 133 a, 133 b, and 133 c, and/or oil equalizing valves 135 a, 135 b, and 135 c such that the oil equalizing operation is ended when a specific condition is satisfied.

Referring to FIG. 5, a difference between a value Td obtained by measuring a temperature of the refrigerant discharged through the output port of the compressor 120 by the first temperature sensor 131 and a value Tpipe obtained by measuring a temperature of the refrigerant discharged to the oil equalizing pipe 114 by the second temperature sensor 133 is generally reduced as the temperature of the outdoor air increases. At this point, the second temperature sensor 133 is connected to a downstream side of the capillary tube 137 provided on the oil equalizing pipe 114. FIG. 5 shows a curve “a” illustrating a state where the oil is exactly collected up to the reference oil level in the compressor 120, a curve “b” illustrating a state where the oil is collected below the reference oil level in the compressor 120, and a curve “c” illustrating a state where the oil is collected above the reference oil level in the compressor 120.

It can be noted that the difference (Td−Tpipe) of the curve “b” is generally higher than that of the curve “a.” The difference (Td−Tpipe) of the curve “c” is generally lower than that of the curve “a.” When the amount of the oil collected in the compressor 120 is equal to or less than the reference oil level, only the refrigerant is directed into the oil equalizing pipe 114. The refrigerant directed into the oil equalizing pipe 114 adiabatically expands while passing through the capillary tube 137. Therefore, the value Tpipe is relatively low and the value (Td−Tpipe) is relatively high.

On the other hand, when the oil is collected above the reference oil level in the compressor 120, the oil existing above the reference oil level is directed into the oil equalizing pipe 114 and thus the Value Tpipe of the temperature measured by the second temperature sensors 133 a, 133 b, and 133 c is relatively high and the difference (Td−Tpipe) is relatively low.

As described above, by utilizing a characteristic where the value (Td−Tpipe) measured in the compressor 120 a that is in the oil excessive state is different from the value (Td−Tpipe) measured in the compressor 120 b that is in the oil shortage state, it can be determined if the oil equalization between the compressors is realized and if the oil equalizing operation stops. This will be described in more detail with reference to FIG. 6.

FIG. 6 is a flowchart illustrating a method of controlling an air conditioner according to an embodiment of the present invention. Referring to FIG. 6, a normal operation step where the air conditioner operates to cool or heat the indoor air is performed (S10). In Step S10, at least one of the compressors 120 a, 120 b, and 120 c operates.

When at least two of the compressors 120 a, 120 b, and 120 c operate (S20), the at least two of the compressors 120 a, 120 b, and 120 c continuously operate for a predetermined time (S30). The predetermined time is a time in which the degree of discharged heat of each of the at least two compressors reaches a reference value. The predetermined time may be 2 hours.

The following will be described on the assumption that the compressors 120 a and 120 b are operating and the compressor 120 c is not operating in Step S10. In addition, it is assumed that the predetermined time is 2 hours. However, the assumptions should not be construed as limitations. The predetermined time can be variously set depending on the air conditioning load or the compressor efficiency.

It is determined if the degrees of discharged heat of the compressors 120 a and 120 b that are operating are greater than a reference value after the compressors 120 a and 120 b operate for 2 hours (S40). In this embodiment, the reference value is 10 degrees, which should not be construed as a limitation.

When the degrees of discharged heat of the compressors 120 a and 120 b are greater than the reference value, both the oil equalizing valves 135 a and 135 b are opened and the oil equalizing operation starts (S50). The oil equalizing valves 135 a and 135 b maintain the opened state for a predetermined time and the oil equalizing operation is continued. In this embodiment, the predetermined time is set to be 90 seconds, which should not be construed as a limitation. The predetermined time may be set considering the number of the compressors that are operating, compression capacity, and/or compression performance such that the oil equalization between the compressors that are operating is sufficiently realized.

When the oil equalizing valves 135 a and 135 b are opened, the oil existing above the reference oil level in the compressor 120 a is directed to the refrigerant common intake pipe 111 through the oil equalizing pipe 114 a and is then dispensed to the compressors 120 a and 120 b that are operating, after which the oil together with the refrigerant is introduced. In this process, the oil existing above the reference oil level is dispensed to the compressor 120 b that is in the oil shortage state. This process is repeated for 90 seconds. In this process, when the oil is excessively supplied to the compressor 120 b above the reference oil level, the oil existing above the reference oil level in the compressor 120 b is dispensed to the refrigerant common intake pipe 111 through the oil equalizing pipe 114 b. As this process is repeated, the oil equalization where the oil is equally dispensed to the compressors 120 a and 120 b is finally realized.

After the above, it is determined if the value (Td−Tpipe) measured in each of the compressors 120 a and 120 b is greater than a predetermined value (S60). When the value (Td−Tpipe) measured in each of the compressors 120 a and 120 b is greater than the predetermined value, this is a case where both the compressors 120 a and 120 b are in the oil shortage state, i.e., a case where the oil cannot be sufficiently supplied up to the reference oil level in the compressors 120 a and 120 b. Therefore, the opened oil equalizing valves 135 a and 135 c are closed (S61) and the oil equalizing operation is stopped (S70).

On the other hand, when the value (Td−Tpipe) measured in each of the compressors 120 a and 120 b is less than the predetermined value, this is a case where the oil is sufficiently supplied to the compressors 120 a and 120 b. Therefore, there is no need to perform the oil equalizing operation and thus the opened oil equalizing valves 135 a and 135 c are closed (S61) and the oil equalizing operation is stopped (S70).

Meanwhile, when the value (Td−Tpipe) measured in one of the compressors 120 a and 120 b that are operating is greater than the predetermined value and the value (Td−Tpipe) measured in the other of the compressors 120 a and 120 b is less than the predetermined value, this means that the oil equalization between the compressors 120 a and 120 b is not yet realized. Therefore, the oil equalizing operation is further performed until a predetermined time passed. At this point, the predetermined time may be set considering durability of the compressor that is in the oil shortage state such that the oil equalization between the compressors 120 a and 120 b is sufficiently realized. In this embodiment, the predetermined time is set to be 4 minutes, which should not be construed as a limitation.

Meanwhile, the value Tpipe is a value that is measured at the downstream side of the capillary tube 137 of the oil equalizing pipe 114. However, when the capillary tube 137 is not provided, the value Tpipe measured in a case where the oil flows along the oil equalizing pipe 114 is different from that measured in a case where the oil does not flow along the oil equalizing pipe 114 even if the value Td is obtained in the oil equalizing pipe 114. Therefore, it will be possible to determine the oil equalizing operation state depending on the value (Td−Tpipe).

According to the air conditioner and method of controlling the air conditioner of the present disclosure, the following effects may be attained.

First, since the oil is supplied from the compressor that is in the oil excessive state to the compressor that is in the oil shortage state, no compressor that is in the oil shortage state may exist.

Second, the damage of components of the compressor, which is caused by the oil shortage, may be prevented.

Third, an oil imbalance phenomenon caused by differences in RPM between the compressors may be prevented.

Fourth, since the oil recovery unit returns the separated oil to the compressors without mixing the oil with the refrigerant, the compression efficiency and coefficient of performance of the compressors may be improved.

The effects as described in the disclosure are not limited to those above, and it will be clear to those skilled in the art that other effects not described can be realized when practicing the following claims.

It will be apparent to those skilled in the art that various modifications and variations can be made. Thus, it should be understood that all embodiments described above are merely exemplary and not restrictive. It is intended that all varied or modified forms derived from the spirit and scope of the appended claims, and equivalents thereof will covered by the scope of the claims to follow. 

1. An air conditioner comprising: a plurality of compressors; a common intake pipe; a plurality of oil equalizing pipes that connect the compressors to the common intake pipe such that oil existing above a reference oil level in the compressors can be transferred to the common intake pipe; and an oil equalizing valve provided on at least two of the oil equalizing pipes to control oil flowing through the oil equalizing pipes, wherein at least two of the oil equalizing valves can be opened together during performance of oil equalizing operation such that amounts of the oil in at least two compressors can be equalized.
 2. The air conditioner according to claim 1, further comprising an oil separator that is provided on an output port side of each of the compressors to return the oil contained in refrigerant discharged from the compressors to the compressors.
 3. The air conditioner according to claim 1, further comprising: a first temperature sensor to measure a temperature of the refrigerant at an output port side of the compressors; and a second temperature sensor to measure a temperature of the refrigerant at an oil equalizing pipe side of the compressors.
 4. The air conditioner according to claim 3, wherein the oil equalizing operation is performed when degrees of discharged heat of all of the compressors are equal to or greater than a reference value.
 5. The air conditioner according to claim 4, wherein, when a difference between a temperature measured by the first temperature sensor and a temperature measured by the second temperature sensor for at least two compressors is equal to or greater than a predetermined value during the oil equalizing operation, the at least two opened oil equalizing valves are closed to stop the oil equalizing operation.
 6. The air conditioner according to claim 4, wherein, when a difference between a temperature measured by the first temperature sensor and a temperature measured by the second temperature sensor for at least two compressors is equal to or less than a predetermined value during the oil equalizing operation, the at least two opened oil equalizing valves are closed to stop the oil equalizing operation.
 7. The air conditioner according to claim 4, wherein, when the oil equalizing operation starts and a predetermined time passes, the at least two opened oil equalizing valves are closed to stop the oil equalizing operation.
 8. The air conditioner according to claim 1, further comprising a plurality of capillary tubes provided on each of the oil equalizing pipes.
 9. The air conditioner according to claim 1, wherein each of the compressors comprises a compressor shell having a first side that is connected to the oil separator to collect the oil and/or refrigerant and a second side that is connected to the corresponding oil equalizing pipe so that the oil/refrigerant existing above a reference oil level can be transferred to the corresponding oil equalizing pipe.
 10. An air conditioner comprising: a plurality of compressors; a common intake pipe; a plurality of oil equalizing pipes that connect the compressors to the common intake pipe such that oil existing above a reference oil level in the compressors can be transferred to the common intake pipe; and a control unit configured to control flow of oil in at least two oil equalizing pipes and to sense temperature at an output port side and an oil equalizing pipe side of at least two compressors that are operating.
 11. The air conditioner according to claim 10, wherein the control unit is configured to stop oil equalizing operation based on a difference between the temperature measured at the output port side and a temperature measured at a downstream side of a capillary tube provided on the oil equalizing pipe in at least two of the compressors that are operating.
 12. The air conditioner according to claim 11, wherein the control unit is configured to stop oil equalizing operation when the difference is equal to or greater than a predetermined value.
 13. The air conditioner according to claim 11, wherein the control unit is configured to stop oil equalizing operation when the difference is equal to or less than a predetermined value.
 14. The air conditioner according to claim 11, wherein the control unit is configured to stop oil equalizing operation when a predetermined time passes after the oil equalizing operation starts.
 15. The air conditioner according to claim 10, wherein the control unit is configured to perform oil equalizing operation when degrees of discharged heat of at least two compressors that are operating are equal to or greater than a reference value.
 16. An air conditioner comprising: a plurality of compressors; a common intake pipe; a plurality of oil equalizing pipes that connect the compressors to the common intake pipe such that oil existing above a reference oil level in the compressors can be transferred to the common intake pipe; and a control unit configured to open oil equalizing valves of at least two oil equalizing pipes connected respectively to at least two compressors that are operating for a predetermined time when degrees of discharge heat of at least two of the compressors that are operating are equal to or greater than a reference value.
 17. The air conditioner according to claim 16, wherein the control unit further configured to measure a temperature of an output port side and a temperature of an oil equalizing pipe side in each of the at least two compressors after a predetermined passes; and the control unit configured to close all of the oil equalizing valves depending on a difference between the measured temperatures.
 18. The air conditioner according to claim 17, wherein, the control unit is configured to close off the at least two opened when the difference is equal to or greater than a predetermined value.
 19. The air conditioner according to claim 17, wherein, the control unit is configured to close off the at least two opened oil equalizing valves when the difference is equal to or less than a predetermined value.
 20. The air conditioner according to claim 17, wherein, the control unit is configured to close the at least two opened oil equalizing valves when of the at least two the oil equalizing valves are opened and a predetermined time passes. 